This project investigates primate biobehavioral development through comparative longitudinal studies of rhesus and capuchin monkeys, with special emphasis on characterizing individual patterns of differential behavioral and physiological responses to environmental novelty and challenge and on determining long-term developmental consequences for individuals of different genetic backgrounds reared in different physical and social environments. During the past year striking differences between rhesus monkey infants whose parents came from wild populations in China and those whose parents were of Northern Indian heritage were found for measures of neonatal reflexes, activity patterns, ability to sustain attention, and temperament during the first month of life. Because both the Chinese derived and Indian derived infants were reared under identical nursery conditions, these differences could not be attributed to differential postnatal experiences. Distinctive patterns of heritability for serotonin, norepinephrine, and dopamine metabolites were found in a large sample of laboratory-born rhesus monkeys reared in comparable environments. Long-term behavioral and physiological effects of differential early rearing experiences were also documented in these monkeys during adolescence and early adulthood. The specific relationships between behavioral expressions of fear and impulsive aggression and characteristic patterns of adrenocortical reactivity and serotonin metabolism previously disclosed for captive rhesus monkeys under laboratory conditions were basically replicated in a wild population of free-ranging rhesus monkeys. Finally, initial studies of the effects of short-term separation in capuchin monkey subjects revealed a range of individual differences in biobehavioral reactions comparable to that observed in rhesus monkeys, despite major differences in fundamental aspects of the mother-infant relationship between these two monkey species.